Recent electronic devices such as smart phones, tablet terminals, laptop computers, portable audio devices, digital cameras, and the like have a touch panel (touch sensor) for operating the electronic devices by touching with a finger.
FIG. 1 is a diagram schematically illustrating a resistive film-type touch panel. The resistive film-type touch panel (hereinafter, simply referred to as a “touch panel”) 100 has a first resistive film 102, a second resistive film 104, a pair of X electrodes 106, and a pair of Y electrodes 108. For the sake of description, X and Y axes are taken in the direction shown in the illustration. The first resistive film 102 and the second resistive film 104 are arranged to face each other with a gap therebetween. The pair of X electrodes 106 are formed along two opposing sides extending in a Y direction of the first resistive film 102. The pair of Y electrodes 108 are formed along two opposing sides extending in an X direction of the second resistive film 104. Wirings (terminals) XP and XN extend from the pair of X electrodes 106, and wirings YP and YN extend from the pair of Y electrodes 108.
When a user touches at any point (contact point) PT, the first resistive film 102 and the second resistive film 104 contact with each other at the point PT. Contact resistance is indicated by Rc. When detecting the X coordinate, a constant voltage (driving voltage) VDRV is applied between the pair of X electrodes 106, and a potential Vx obtained by dividing the constant voltage VDRV by resistors Rx1 and Rx2 is generated at the contact point PT.Vx=VDRV×Rx1/(Rx1+Rx2)
Since Rx1+Rx2 is panel impedance (referred to as panel resistance Rx) between the wirings XP and XN and is a value unique to the panel, Vx=VDRV×Rx1/Rx.
Since the resistance Rx1 has a value corresponding to the X coordinate, the measured voltage Vx indicates the X coordinate. When both the wirings YP and YN are set to high impedance, the potential Vx of the contact point is observed through the second resistive film 104 in the potentials of the wirings YP and YN.
When detecting the Y coordinate, the constant voltage VDRV is similarly applied between the pair of Y electrodes 108, and the potential Vy of the first resistive film 102 is measured.Vy=VDRV×Ry1/(Ry1+Ry2)
Recently, a touch panel compatible with multi-touch has been required. For example, a multi-touch detection technique in a resistive film-type touch panel is provided. FIG. 2 is an equivalent circuit diagram of a touch panel when multi-touch occurs. Rc1 and Rc2 indicate contact resistances, and Ry2 indicates impedance between two points P1 and P2 of the second resistive film 104.
As the distance between the two points P1 and P2 of the multi-touch is longer, the combined impedance of the first resistive film 102 and the second resistive film 104 decreases. Therefore, coordinates of the two points may be detected by detecting the combined impedance Zx. In order to detect the combined impedance Zx, a current (panel current Ix) flowing through the panel is detected. This also applies to the Y coordinate.
The panel current Ix is given by the following equation.Ix=VDRV/Zx Zx=Rx1+Rx3+Rx2//(Rc1+Ry2+Rc2)
The above equation indicates combined resistance of parallel resistors.
Rx1+Rx2+Rx3=Rx is established.
FIG. 3A and FIG. 3B are diagrams illustrating a relationship between a distance Δx between two points and a panel current Ix. Here, this relational equation is schematically indicated by a straight line, but actually, it is a more complicated curve.
The state of one point touch corresponds to zero of a distance Δx between two points. The impedance Zx of the panel when the distance Δx between the two points is zero is Zx=Rx1+Rx2+Rx3=Rx, it is equal to that in a non-touch state, and a panel current Ix0 at that time is given by Ix0=VDRV/Rx.
When the distance Δx between the two points increases, the panel current Ix increases from Ix0. That is, the current Ix0 stably flows irrespective of the presence or absence of touch, and it is referred to herein as a constant current or a constant component.
Recently, the resistance of the resistive film has been reduced and the panel resistance Rx has been reduced. FIG. 3A illustrates a case where the panel resistance Rx is relatively large, and FIG. 3B illustrates a case where the panel resistance Rx is relatively small. The panel current Ix is normalized so that the maximum value becomes the same for easy comparison. As can be seen from the comparison of FIG. 3A and FIG. 3B, as the panel resistance Rx decreases, the sensitivity of the panel current Ix to the change of the distance Δx between the two points decreases.
From another point of view, when the panel current Ix of FIG. 3A and FIG. 3B is quantized with the same current resolution, in FIG. 3B, the effective resolution of a fluctuation component depending on the distance between the two points included in the panel current Ix is significantly reduced. This problem should not be regarded as a general perception of those skilled in the art.